Near-Infrared Spin-Flip Luminophores with Earth-abundant Metal Ions 2.0 (NIR-SPINFLIP 2.0)

发光金属络合物是有机发光二极管等许多应用领域的重要组成部分。目前，这些技术主要依赖于昂贵的金属前驱体，而基于廉价和富含地球的金属离子的高效发光材料到目前为止还很少。由于电子激发金属络合物的许多非辐射衰减路径的普遍存在，通常导致低发光效率，因此开发在近红外(NIR)光谱区域发射的材料尤其具有挑战性。该项目将开发和调整基于地球上丰富的具有d3和d2电子组态的金属离子(主要是钒和钼离子)的分子自旋翻转近红外发射器。对于3D2钒(III)和3D3钒(II)金属离子，强配体将以准八面体配位几何构型配位，以增加钒的本征弱配位场分裂，并将组态间配位场激发态的能量推到发射自旋反转态以上。这将阻止后台系统间交叉(Back-ISC)到这些暗状态。在配体上安装溴或碘取代基以利用重原子效应将有利于ISC从最初的激发态到自旋翻转态的路径。为了提高自旋翻转发射的辐射率，异构性、非中心对称的络合物起到了缓解拉波特规则的作用。配位体的系统氢化和CH/Cd泛音测量有助于通过多声子弛豫来理解和减少非辐射衰变。通过这些组合策略，我们将开发一种基于钒(III/II)氧化还原对的氧化还原可切换发光(RSL)平台，其中两个氧化态都显示出与传统氧化还原可切换开/关发射体不同的发光。对于已经具有固有的大配位场分裂和自旋轨道耦合常数的4d金属离子钼(III)，基于弱场配体的互补策略和在自旋翻转态上方安装电荷转移态作为天线将被开发出来，以实现有效的自旋翻转态布居。ISC速率将由泵浦探测光谱确定。步进扫描傅里叶变换红外光谱将探测自旋-翻转态的结构，而近红外光致发光将在不同的环境下用静态光谱和时间分辨光谱进行研究。结果将加深我们对过渡金属络合物激发态衰变过程的理解，并进一步系统地改进涉及金属络合物的许多关键技术，如光学通信或近红外生物医学成像。

Luminescent metal complexes are important building blocks in many applications such as organic light-emitting diodes. Currently, these technologies mainly rely on expensive metal precursors, while efficient luminophores based on cheap and earth-abundant metal ions are rare up to now. The development of materials that emit in the near-infrared (NIR) spectral region is particularly challenging due to the prevalence of many non-radiative decay pathways of electronically excited metal complexes, usually leading to low luminescence efficiencies. This project will develop and tune molecular spin-flip NIR emitters based on earth-abundant metal ions with d3 and d2 electron configurations (primarily vanadium and molybdenum ions). For the 3d2 vanadium(III) and 3d3 vanadium(II) metal ions, strong ligands will be coordinated in a pseudo-octahedral coordination geometry to increase the intrinsically weak ligand field splitting of vanadium and to push the energy of the interconfigurational ligand field excited states above the emissive spin-flip states. This will block the back-intersystem crossing (back-ISC) to these dark states. Installing bromine or iodine substituents at the ligands to exploit the heavy-atom effect will favor the ISC path from initially excited states to the spin-flip states. To increase the radiative rate for the spin-flip emission, heteroleptic, non-centrosymmetric complexes serve to mitigate Laporte’s rule. Systematic deuteration of the ligands and CH/CD overtone measurements serve to understand and decrease non-radiative decay via multiphonon relaxation. With these combined strategies, we will develop a redox-switchable luminescence (RSL) platform based on the vanadium(III/II) redox couple where both oxidation states exhibit luminescence in contrast to conventional redox-switchable on/off emitters.For the 4d metal ion molybdenum(III), which already possesses intrinsically large ligand field splittings and spin-orbit coupling constants, complementary strategies based on weak-field ligands and the installation of charge-transfer states slightly above the spin-flip states as antennas for efficient population of the spin-flip states will be developed.ISC rates will be determined by pump-probe spectroscopy. Step-scan FTIR spectroscopy will probe the structure of the spin-flip states, while NIR photoluminescene will be investigated by static and time-resolved spectroscopy in various environments. DFT calculations augmented by multireference calculations complement the spectroscopic studies.The results will deepen our understanding of excited state decay processes of transition metal complexes and further the systematic improvement for many key technologies involving metal complexes such as optical telecommunications or NIR biomedical imaging.